This invention generally relates to micro-electromechanical system (MEMS) switches, and more particularly, to a hinge type MEMS switch and a method of fabricating the same using current state of the art semiconductor fabrication processes, such as a CMOS process.
Switching operations are a fundamental part of many electrical, mechanical and electromechanical applications. MEMS switches have drawn considerable interest over the last few years, leading to the design and development of a variety of products using MEMS technology that have become widespread in biomedical, aerospace, and communications systems applications.
Conventional MEMS typically utilize cantilever switches, membrane switches, and tunable capacitor structures, as described, e.g., in U.S. Pat. No. 6,160,230 to McMillan et al., U.S. Pat. No. 6,143,997 to Feng et al., U.S. Pat. No. 5,970,315 to Carley et al., and U.S. Pat. No. 5,880,921 to Tham et al. MEMS devices are manufactured using micro-electro-mechanical techniques and are mainly used to control electrical, mechanical or optical signal flows. Such devices, however, present many problems because their structure and innate material properties require that they be manufactured in lines that are separate from conventional semiconductor manufacture processing. This is usually due to materials and processes which are incompatible and which cannot be integrated within existing semiconductor fabrication lines.
Implementing MEMS (micro-electromechanical systems) switches for semiconductor applications has many advantages, such as: (1) low insertion loss, (2) low or no DC power consumption, (3) high linearity, and (4) broad bandwidth performance. However, it must be provided with a low actuation-voltage switch and must not suffer from stiction, that is, the inability to restore the switch to its original state when desired. A conventional cantilever type switch, as shown in FIGS. 1A-1B or a membrane type switch, as illustrated in FIGS. 2A-2B typically requires 10 to 100 V operating voltage, which is unsuited for integration with state-of-the-art integrated circuits.
Referring back to the aforementioned U.S. Pat. No. 6,143,997, to Feng at al., and in particular, to FIG. 1A illustrated therein, a prior art cantilever switch is shown in a resting position with the cantilever portion a distance hA away from an RF transmission line creating an off state, since the distance hA prevents current from flowing from the cantilever to the transmission line below it. To turn the switch on, as shown in FIG. 1B, a large switching voltage, typically in the order of 28 Volts, is necessary to overcome physical properties and bend the metal down to contact the RF transmission line. In the energized state, with the metal bent down, an electrical connection is created between the cantilever portion and the transmission line. Thus, the cantilever switch is on when it remains in the excited state.
Referring to FIGS. 2A-2B, Feng et al. show a membrane switch, respectively, in a resting and energized position. When the membrane switch remains in its resting position, current is unable to flow from the membrane to an output pad and the switch is turned off. Similar to the cantilever switch, a high actuation voltage, typically 38 to 50 volts, is necessary to deform the metal and activate the switch. In the excited state, the membrane is deformed to contact a dielectric layer on the output pad, thereby electrically connecting the membrane to the output pad to turn the switch on. This design also requires a relatively high voltage.
In contrast to cantilever switches, the switching action for hinge type MEMS switches requires very low actuation voltage, typically less than 3 volts, mainly because they lack mechanical bending action. U.S. Pat. No. 6,143,997 to Feng et al. describes this type of switch. Referring to FIG. 3, the switch pad moves up and down freely along hinge bracket 22. In a relax state, as shown in FIG. 3A, the pad is attracted by a lower electrode 20, forcing it to stay at the lower level. In an energized state, as illustrated in FIG. 3B, the pad is attracted by the top electrode 30, moving it to the top level. It is worth noting that the metal pad, i.e., rigid movable element 17, makes contact with dielectric 32 when in the upward position, and with, e.g., dielectric 18, when in a downward position. Thus, the MEMS switch described only operates as an RF switch, adequate for a high frequency environment, wherein pad 17 and electrodes 20 act as the metal plates of a capacitor separated by a dielectric layer which acts as an open circuit for a DC voltage, but as a short for an AC voltage. Furthermore, the process steps to fabricate a hinge-type MEMS switch are not described by Feng et al., and no reference is made on how to integrate this type of MEMS switches alongside with back-end-of-the-line (BEOL) metal interconnects of a conventional semiconductor chip.
Another type of MEMS switch is described by L. Frenzel, in the article “MEMS Switch Puts SoC Radios on the Cusp”, Electronic Design, p. 29, Jun. 9, 2003, that uses a combination of thermal and electrostatic actuation. These devices have been used for band and circuit reconfiguration in a multi-band/multi-mode RF system. In order to change the state of the switch, each time 20 mA current must be applied, which is not practical for a CMOS chip environment.
More and more MEMS switches are emerging for RF applications. For example, STMicroelectronics describes a combination of thermal and electrostatic actuation type MEMS switches for mode of operation and circuit configurations designed for multi-mode and multi-band RF system applications. Such switches also require 20 mA of current to heat the device and allow it to switch. High-currents of this magnitude are not suitable for CMOS applications. To date, conventional MEMS switches are not CMOS compatible because: (1) they are difficult to integrate using MOS process steps and, (2) they require a high-current and high actuation operation voltages.
Thus, there is a need in industry for an improved MEMS, particular, a hinge-type MEMS switch that is suited for a wide range of semiconductor switching applications, spanning from RF, optical, mechanical, package, cooling, and extending to include CMOS circuit applications, and which are characterized by having a low actuation voltage (less than 3 V) and which can easily be integrated within conventional integrated circuit (IC) manufacturing lines.